How to export a 3D model of just a particular frame?
Go to the frame you want and apply your modifiers. Including your cloth modifier. If you have Interface: Modifier Tools Add-on enabled you can press Apply All. If not then you can press the little down arrow and Apply. Or select a modifier and press Ctrl+a. Remember to start from the top.

Now your cloth is frozen and can be used as any 3D-object and exported from File and Export.